Static and dynamic wetting of aligned nanorod arrays

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Abstract

A systematic study of the static and dynamic wettability of nanorod array surfaces prepared by glancing angle deposition is presented. The static wettability of the aligned Si nanorod array surfaces was investigated by the sessile drop method. As-prepared nanorod samples were hydrophilic, while after hydrofluoric acid treatment or fluorocarbon coating, they became hydrophobic. The contact angle ¸ was oosuccessfully tailored from superhydrophilic (¸ = 3) to superhydrophobic (¸ = 170) by controlling the nanorod height, rod-rod separation, and chemical treatment. For the as-prepared samples, a wetting transition from a porous surface to a hemi-wicking surface was observed as the nanorod height increases. For HF treated samples, a transition from rough surface to composite surface was also observed. The transitions took place at the same critical nanorod height about 150 nm. The superhydrophilic and superhydrophobic behaviors as well as the wetting transitions are well interpreted by Wenzel’s law and Cassie’s law. In the wetting experiment, nanorods bundled together to form patterns. This nanocarpet phenomenon presents a challenge for high aspect-ratio nanostructures to be used in liquid environments. Both the morphology of the pattern and the physical origin were investigated theoretically and experimentally. The bundling was caused by unbalanced capillary forces acting on the nanorods either during the spreading process or the drying process. Thus controlling the wetting and dewetting process may reduce the bundling. Furthermore, the mechanical stability of several nanorod structures was compared and a capping layer method was proposed to minimize the nanorods from bundling. Dynamic spreading and capillary rise experiments were investigated by fast CCD video imaging. During the spreading of water droplets on the nanorod surface, water also penetrates into the nanorod channels and transports faster than the apparent contact line. The contact line dynamics was similar to those on flat surfaces due to the spreading of the precursor film inside the nanorod channels ahead of the contact line. And the evolution of the precursor film follows Washburn’s law. The scaling of these dynamics is almost not affected by the nanorod heights. For the capillary rise, the contact line and precursor dynamics are more complicated and they are affected by the sample size as well as the gravity effect.